Heat-resistant resins, which have excellent physical and chemical properties, have been molded in many different shapes and used in various fields. Particularly in the most advanced high-tech fields such as aerospace technology, electronics and the like, heat-resistant resins have been useful because of their heat-resistant property, mechanical strength, dimensional stability and chemical stability.
However, heat-resistant resins require special treatments such as a long heat treatment at high temperature to harden and form the resins into an imide. Molding procedures applied to general plastic materials are not applicable to heat-resistant resins since the simple heat treatments of the molding procedures for general plastic materials do not melt and harden the resins, so that it has been difficult to process heat-resistant resins so as to fully utilize the properties of the resins. In other words, it has been extremely difficult to provide a molding technology for heat-resistant resins.
Tubes made from plastic high polymer materials, rubber, and the like have been developed as conventional seamless tubes with thin tube wall thickness, and their uses are diverse. Methods used for manufacturing these tubes include an injection method, an inflation method, and the like, and it is extremely difficult to manufacture seamless tubes having an even tube wall thickness between several .mu.m and dozens of .mu.m with these methods. The seamless tubes having thin tube walls, which are manufactured by the inflation method, are further thinned, in some cases, through methods of drawing the tubes in a longitudinal direction or passing them between pressure rollers. However, tubes manufactured by these methods still have uneven tube wall thickness.
An example of a conventional method of manufacturing a tube from polyimide resin includes the following steps:
treating the surface of a tetrafluoroethylene-hexafluoropropylene copolymer film with a corona discharge; PA1 heating and laminating a polyimide film on the surface of the copolymer film, thus manufacturing a tape having a two-layer structure; PA1 wrapping a core with the tape at a uniform thickness; PA1 heating and melting the tape wrapped around the core; and PA1 extracting the core. PA1 pouring polyamide acid solution into a molding pipe such as a glass pipe, stainless pipe, or the like with a smooth internal surface; PA1 holding the molding tube in a vertical position; PA1 dropping a bullet-like object through the solution by its own weight, thereby forming a hole inside the solution; PA1 heating and drying the solution inside the molding pipe, thus causing it to become imide by imide reaction and forming a tube; and PA1 extracting the tube from the molding pipe. PA1 treating the tube around the core to give it strength as a tube; and PA1 separating the tube from the core. PA1 coating a precursor solution of a tube material on the surface of the core at a thickness greater than the final tube wall thickness; PA1 passing a metallic die along the outside of the core by utilizing the resistant force of the viscosity of the precursor solution without restricting the die or the core, thus forming the tube around the core; PA1 drying, hardening, and heating the tube or extracting a solvent from the tube to provide strength to be tube; PA1 separating the tube from the core, thus providing a tube with a uniform tube wall thickness of 3-300 .mu.m. PA1 coating a precursor solution on the surface of the core at a thickness greater than the final tube wall thickness by dipping the core into the precursor solution, applying the precursor solution with a brush, or using general application methods such as a flow coating method; and PA1 passing and dropping the metallic die along the outside of the core by its own weight; PA1 or alternatively, pulling the core such as with a thread or string while the metallic die is fixed with or without certain flexibility.
The tubes manufactured by this conventional method, however, cannot be used at a temperature higher than the resisting temperature of tetrafluoroethylene hexafluoropropylene copolymer, and the heat-resistant property of polyimide cannot be fully utilized in the method. In addition, the tubes manufactured by wrapping tape around the core have a spiral structure, and their tube wall thickness is usually uneven.
One example of a method of manufacturing polyimide tubes with uniform tube wall thickness is disclosed in Japanese Published Unexamined Patent Application No. Hei 1-156017. The polyimide tube is manufactured by this method in the following steps:
However, when the polyamide acid solution has high viscosity, the dropping speed of the bullet-like object slows as the polyamide acid solution tends to gather toward the end of the molding pipe. As a result, in the above method, the tube wall thickness tends to be uneven, and there is an upper limit on the diameter of the tubes. Manufacturing costs are also extremely high. Moreover, even if this method can provide polyimide tubes with uniform tube Mall thickness in some experiments, it is difficult to mass-produce the tubes. The final treatments such as drying, hardening and heating the tubes are also difficult.
There has been research on a method of manufacturing a tube with uniform tube wall thickness by using a casting method. Casting methods include a method of dipping a core into a liquefied tube material or precursor material of the tube material and lifting the core from the liquid tube or the precursor material (dipping method), and also a method of spraying the liquefied or powdered precursor material on the surface of a tube, and the like. However, there is an upper limit on the viscosity of the precursor materials, and it is impossible to manufacture a tube with even tube wall thickness as the viscosity of the precursor becomes high. In these methods, the material coated on the core is dried, hardened and reacted, and is separated from the core, thus providing a tube. However, the tube obtained from these processes cannot have a uniform tube wall thickness.
A method of setting a metallic die outside a core formed with a tube material or its precursor material at a certain distance and passing the die along the core can be used for providing a tube with an even tube wall thickness. However, it is hard to keep the core and metallic die parallel even though such parallelism is necessary for providing a tube with a uniform tube wall thickness. It is actually impossible to control and maintain the parallelism and eccentricity between a long core and metallic die within .+-.10% or less than .+-.5% of the required level of parallelism and eccentricity.
Heat-resistant seamless tubes with uniform tube wall thickness can be used in conveyor belts for high-performance precision instruments, copiers, picture processing films for laser beam printers, etc., functional materials for precision machines, and the like. In manufacturing tubes for these purposes, the outside surfaces of the tubes should be as smooth as possible, and the inside surface of the tubes should possess the same texture as the core surface.
Even though there have been some proposals for improving seamless tubes such as in Japanese Published Unexamined Patent Application No. Hei 3-180309 and Japanese Published Unexamined Patent Application No. Hei 3-261518, further improvements are required.